Slurry for slicing silicon ingot and method for slicing silicon ingot using the same

ABSTRACT

The invention is a slurry for slicing a silicon ingot, containing a basic material, such as an alkali metal hydroxide, abrasive powder and water, in which the slurry contains the basic material in an amount of from 2 to 6% by mass and glycerin in an amount of from 25 to 55% by mass, based on a total mass of components of the slurry excluding the abrasive powder.

TECHNICAL FIELD

The present invention relates to a slurry used upon slicing a siliconingot, for example, for producing a wafer for a solar cell, and a methodfor slicing a silicon ingot using the same.

BACKGROUND ART

For slicing a silicon ingot, conventionally, a multi-wire saw has beenused that is capable of slicing a large number of wafers at a time witha relatively small kerf loss. FIG. 1 shows a device constitution of abasic multi-wire saw for slicing a silicon ingot for producing a waferfor a solar cell. In a multi-wire saw 10 shown in the figure, numeral 1denotes a silicon ingot, which is fixed by adhesion to a working plate2, and in general, a material having a cross section of about 150 mmsquare and a length of about 400 mm is relatively frequently used.

Numeral 3 denotes a wire fed from a wire feeding mechanism 4, which iswound and suspended on two wire guide rollers 5 at a pitch of about from0.3 to 0.4 mm and then wound up by a wire winding mechanism 6. A pianowire having a diameter of 0.16 mm is generally used as the wire 3. Thewire 3 is fed at a speed of about 600 m/min while the feeding mechanism4, the two wire guide rollers 5 and the winding mechanism 6 are drivenwith motors (which are not shown in the figure) that are synchronouslycontrolled, and a prescribed tension is applied to the wire 3 bycontrolling the position of a tension roller 7. Slurry is applied to thewire 3 from a slurry agitating and feeding tank 8 through a slurryapplying head 9. In this state, the ingot 1 fixed by adhesion to theworking plate 2 is fed downward.

The slicing operation of the silicon ingot 1 using the wire saw 10 iseffected in such a manner that a slicing slurry containing abrasivepowder is fed between the running wire 3 and the ingot 1, and theabrasive powder are pressed onto the ingot 1 and simultaneously arerotationally moved with the wire 3, whereby microcracks are formed inthe surface layer of the ingot 1 to scrape away the surface layer assilicon fine powder. In the slicing operation of the silicon ingot 1 inthis manner, it is demanded to decrease the kerf loss and to decreasethe thickness of the wafers in order to improve the yield of the wafersand to reduce the cost of materials of the wafers.

FIG. 2 is an enlarged view of the slicing part of the silicon ingot 1using the multi-wire saw 10 and shows the relationship among theparameters upon slicing the silicon ingot 1. FIG. 3 is a schematicdiagram showing the force applied to the wire in the slicing groove ofthe silicon ingot 1. In FIGS. 2 and 3, the following empiricalexpressions have been generally known in the art as showing therelationship among the feeding speed V of the silicon ingot 1, thefeeding speed U of the wire 3, the slicing resistance P, thedisplacement δx of the wire 3 in the direction perpendicular to theslicing direction, the displacement δy of the wire 3 in the slicingdirection, and the tension T of the wire 3.P∝V/U  (1)δx∝c P/T  (2)δy∝P/T  (3)

Upon feeding the slurry containing abrasive powder 11 to the interfaceto be sliced with the wire 3, the wire 3 is deflected to form thedisplacement δy, and the slicing resistance P is formed. The slicingresistance P is gradually increased but reaches constant at a prescribedvalue. At the slicing interface, the abrasive powder 11 are notuniformly dispersed, and the displacement δx of the wire 3 is formed byacting a force proportional to the slicing resistance P in the directionperpendicular to the slicing direction.

In the case where the value of δx is increased, wafers obtained byslicing the silicon ingot 1 suffer board warpage, unevenness inthickness and minute surface irregularities, so as to deteriorate thequality of the wafers. It may be suggested from the expression (2) thatthe slicing resistance P should be decreased in order to decrease δx.

Accordingly, the feeding speed V of the silicon ingot 1 may bedecreased, or the feeding speed U of the wire 3 may be increased, asunderstood from the expression (1), but in the case where the feedingspeed V of the ingot 1 is decreased, the slicing time is prolonged todeteriorate the production efficiency. In the case where the runningspeed U of the wire is increased, the consumption amount of theexpensive wire is increased to increase the running cost for slicing. Inorder to decrease the kerf loss, it is necessary to decrease thediameter of the wire 3, but the breaking strength of the wire 3 isdecreased thereby to provide necessity of decreasing the tension Tapplied to the wire 3. In the case where the tension T is decreased, thedisplacement δx of the wire 3 is increased as understood from theexpression (2), and thus the wafers are deteriorated in quality ashaving been described above.

Microcracks remain in the surface layer of the wafers after slicing.Upon producing a solar battery cell by processing the wafers, it isnecessary to remove firstly the damaged layer by etching, and thus thewafer thickness is further decreased after slicing the ingot. In thecase where the wafer thickness becomes smaller, the damage rate of thewafers in the transporting and processing steps of the wafers isincreased. The depth of the damaged layer is about 10 μm in theconventional techniques, which impairs reduction in thickness of thewafers.

As a method for removing the problem, fundamental studies have been madefor a wire saw using slurry containing an alkaline aqueous solution andabrasive powder. It has been reported that in the case where a chemicaldissolution function is imparted to a processing liquid upon slicing asilicon ingot, the resistance on moving the wire (which is hereinafterreferred to as a wire pulling resistance) is decreased, and the crackdepth on the surface layer of the ingot is decreased (see, for example,in Non-patent Document 1).

FIG. 4 is a conceptual diagram for describing the effect obtained byimparting a chemical function to the slurry. In the figure, symbol A1denotes the state where a neutral slurry is used, and A2 denotes therelationship between the time and the wire pulling resistance F of thewire 3 in this case. Symbol B1 denotes the state where alkaline slurryis used, and B2 denotes the relationship between the time and the wirepulling resistance F of the wire 3 in this case.

The abrasive powder 11 in the slurry are pressed on the ingot 1 androtationally moved with the wire 3, whereby cracks 12 formed one afteranother on the surface of the ingot 1 are connected to each other, andfine powder of silicon is formed at regions where they reach the surfaceof the ingot 1 and discharged from the slicing groove with the abrasivepowder rotationally moved and the flowing slurry liquid. The formationof fine powder means elimination of cracks. It has been studied thereinthat in the case where alkaline slurry is used, the cycle time fromformation to elimination of cracks is shortened as understood from thecomparison between A2 and B2, and fine powder is formed on anddischarged from the surface of the ingot 1 before application of a largeremoving power (wire pulling resistance F) at the slicing interface.

In FIG. 2, the tension T is applied to the wire 3, whereby thedisplacement δx of the wire 3 in the direction perpendicular to theslicing direction is to be a prescribed value, but the wire pullingresistance F is added thereto in the portion of the wire 3 on the sidewhere the wire is withdrawn from the ingot 1. In the case where the wirepulling resistance F is decreased, the force applied to the wire 3 isdecreased. The diameter of the wire 3 is decreased through movement ofthe wire 3 from the feeding side to the winding side of the wire sincethe wire is continuously abraded with the abrasive powder. In the casewhere the wire pulling resistance F is decreased, the abrading force isalso decreased to suppress reduction in diameter of the wire 3. Thereduction of the wire pulling resistance F applied to the wire 3 and thereduction of the abrading amount of the wire provide capability ofreducing the diameter of the wire, and thus the kerf loss can bedecreased.

On the line of the aforementioned approach, such a single wire saw hasbeen proposed that uses an alkaline slurry of pH 9 or more at from 30 to80° C. or a acidic slurry of pH of from 3 to 6 at from 25 to 65° C. (seePatent Document 1).

Such a method of slicing a material to be processed has been proposed inthat an etching liquid containing no abrasive particle is coated on awire, and the temperature of the etching liquid is increased to 50 to60° C. by frictional heat formed between the wire and the material to beprocessed (see Patent Document 2).

Non-Patent Document 1:

Electronics yo Kessho Zairyo no Seimitsu Kako (Precision Processing ofCrystalline Materials for Electronics), published on January 30, Showa60 (1980), by Science Forum Inc.

Patent Document 1: JP-A-2-262955

Patent Document 2: JP-A-2-298280

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Upon slicing a silicon ingot for producing wafers for solar cells, it isdemanded to slice into several thousands of wafers at a time from thestandpoint of allowable production cost. The sliced wafer is used afterremoving the slicing damaged layer on the surface thereof by etching,but the step of finishing the surface by polishing as in wafers forsemiconductors may not be added from the standpoint of avoiding increasein production cost. Accordingly, it is necessary to satisfy demandedvalues of undulation and variation in thickness of the wafers only byslicing an ingot.

The method of coating an etching liquid to a wire as disclosed in PatentDocument 2 requires strict management of the contact force of the wireto the material to be processed and the temperature at the slicinginterface, and the method can be practiced with a single wire system butcannot be realized upon slicing with a large number of wires.

In the slicing operation of an ingot using acidic or alkaline slurrycontaining abrasive powder as disclosed in Patent Document 1, suchfactors are important as control of the reaction of silicon with acid oralkali and maintenance of the dispersibility of the reaction productsand the abrasive powder in the liquid. The wire forms a slicing groovethrough progress in the slicing direction, and since the slurry iscontinuously supplied to the slicing groove, dissolution in thedirection perpendicular to the slicing direction (i.e., the thicknessdirection of the wafer) proceeds in the slicing initiation part ascompared to the slicing termination part.

Upon slicing a silicon ingot for producing wafers for solar cells, thewafer thickness may not be corrected by additional polishing, and thusthe demanded value of wafer thickness cannot be satisfied when theetching effect is excessive. Furthermore, a large amount of slurry isnecessarily used for slicing out several thousands of wafers from aningot, which is realized by circulating the slurry between the agitationtank and the slicing part. In the case where aggregation occurs due todeterioration in dispersibility of reaction products with silicon andthe abrasive powder, feeding of the abrasive powder to the slicinginterface is imparted, whereby the wire pulling resistance of the wireis increased to break the wire, and the slicing resistance is increasedto increase the displacement of the wire in the direction perpendicularto the slicing direction, which causes undulation and steps on thesurface of the wafer.

As a result of a slicing experiment of a silicon ingot of 150 mm squarewith 50 wires using an alkaline aqueous solution slurry adjusted to pH12 at 60° C. containing abrasive powder, the displacement of the wire inthe direction perpendicular to the slicing direction was large, theadjacent wires slice the same groove, and the wire was broken afterslicing in a depth of 50 to 70 mm. It was found as a result of analysisthat this is because the abrasive powder were not sufficiently suppliedto the slicing interface due to aggregation of the reaction products andthe abrasive powder, whereby the slicing resistance and wire pullingresistance of the wire were increased. There were some cases wherecompletely no abrasive particle was supplied to the slicing interface,and the temperature at the slicing part of the ingot was rapidlyincreased due to direct friction between the ingot and the wire to breakthe wire.

Upon slicing an ingot using slurry having a chemical function andcontaining abrasive powder, it is necessary to determine such a slurryformulation that realizes control of the chemical reaction andmaintenance of dispersibility of the reaction products and the abrasivepowder in the liquid. The invention is to solve the aforementionedproblems, and an object thereof is to provide such a slicing slurry inthat upon slicing a silicon ingot, the wire pulling resistance of thewire is small, and the ingot can be sliced while suppressing unevennessin thickness, minute irregularities and damages on the surface layer ofthe wafer, and to provide a method for slicing a silicon ingot using thesame.

Means for Solving the Problems

The invention relates to slurry for slicing a silicon ingot, containingabrasive powder, a basic material and water, the slurry containing thebasic material in an amount of from 2 to 6% by mass and glycerin in anamount of from 25 to 55% by mass, based on a total mass of components ofthe slurry excluding the abrasive powder.

Advantage of the Invention

The slurry for slicing a silicon ingot of the invention is an aqueousslurry containing a basic material, glycerin and abrasive powder, andthe components are contained in suitable amounts, respectively, based onthe mass of the total liquid components of the slurry, whereby thechemical action with silicon can be controlled, and the dispersibilityof reaction products and the abrasive powder in the liquid can bemaintained. Accordingly, synergistic effects of chemical action andphysical action upon slicing a silicon ingot can be enjoyed, and theremoving power of silicon at the slicing interface (i.e., the wirepulling resistance of the wire) can be decreased with the demandedquality of wafers for solar cells maintained, whereby an ingot can bethinly sliced into wafers by decreasing the kerf loss and the slicingdamage with a thin wire to attain reduction in cost of wafers.

Other objects, characteristics, viewpoints and advantages than thosedescribed above will be apparent from the following detailed descriptionof the invention referring to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic constitution of a multi-wire sawfor slicing a silicon ingot used in the invention.

FIG. 2 is an enlarged view of a slicing part of a silicon ingot usingthe multi-wire saw.

FIG. 3 is a schematic diagram showing the force applied to the wire inthe slicing groove of the silicon ingot.

FIG. 4 is a conceptual diagram for describing the effect obtained byimparting a chemical function to the slurry.

DESCRIPTION OF SYMBOLS

-   1 ingot-   2 silicon ingot feeding mechanism-   3 wire-   4 wire feeding mechanism-   5 roller-   6 wire winding mechanism-   7 tension controlling roller-   8 slurry agitating and feeding tank-   9 slurry coating head-   10 multi-wire saw

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

Upon slicing a silicon ingot with a multi-wire saw, it is necessary tosupply abrasive powder continuously to the slicing interface in asuitable amount. A wire is used as means for conveying the abrasivepowder to the slicing interface, and a liquid is necessarily used as amedium for dispersing and carrying the abrasive powder on the wire andfor reducing the frictional force among the wire, the abrasive powderand the ingot and effecting cooling at the slicing interface. It isnecessary to manage the viscosity of the liquid within a certain range.In the case where the viscosity is low, the necessary amount of theabrasive powder cannot be carried on the wire, and in the case where theviscosity is high, on the other hand, the liquid cannot permeate to theslicing interface, and thus the necessary amount of the abrasive powdercannot be supplied to the slicing interface similarly. Furthermore, theliquid pressure at the slicing part is increased to cause a forceseparating the wafers that are being sliced, and the flexure stress atthe slicing interface is increased by the force to break the wafers. Itis also important to disperse the abrasive powder in the liquid. In thecase where the abrasive powder are aggregated, the abrasive powder areaccumulated at the inlet, through which the wire enters from the outsideof the ingot to the slicing part, whereby the amount of the abrasivepowder supplied to the slicing interface is decreased, and moreover, thewire pulling resistance of the wire is increased to break the wire.

It is important to select such a liquid that has a prescribed viscosity,does not decrease the chemical action of the basic material, and doesnot cause increase in viscosity and aggregation of the abrasive powderdue to silicate and silica as reaction products of the basic materialand silicon. As a result of various investigations, it has been foundthat glycerin is suitable as the liquid having these properties.Glycerin has a suitable viscosity, has a large polarity to suppressdecrease of the zeta potential in the slurry liquid due to formation ofsilicate and silica, and has good affinity with water to suppress theincrease in viscosity of the slurry due to hydration and gelation ofsilicate and silica with water.

In the case where the amount of the basic material is small, theproportion of silica in the silicate is increased to accelerate decreasein zeta potential and increase in viscosity, in addition to the failurein exertion of the chemical action. In the case where the amount of thebasic material is large, on the other hand, the etching action becomestoo large, and thus the dissolution amount of the wafer at the slicinginitiation part exceeds the limit. The amount of hydrogen formed throughreaction with silicon becomes excessive to increase the amount of gasbubbles in the slurry at the slicing interface, which considerablydecreases the slicing speed due to a large amount of abrasive powderdepletion parts, whereby in some cases, the liquid lubricating contactamong the wire, the abrasive powder and the silicon ingot is lost tobreak the wire due to the frictional force thus increased.

It is important to manage the amount of the basic material, and it hasbeen found that there is an optimum value therefor with respect to themass ratio of glycerin in the slurry liquid components.

In the component ratios of the first slurry for slicing a silicon ingotof the invention, while maintaining the demanded quality of wafers forsolar cells, the removing power of silicon at the slicing interface (thewire pulling resistance of the wire) can be decreased, and control ofthe chemical action of the slurry with silicon and dispersibility of thereaction products and the abrasive powder in the liquid can bemaintained. It has been found that the expected advantages cannot beobtained outside the component ratios.

Although various kinds of alcohol, amine, ether, polyethylene glycol andthe like have been studied other than glycerin, there has been nomaterial that has a stable viscosity, causes no aggregation of theabrasive powder, and maintains the suitable chemical action, under theslicing conditions where oxidation reaction of silicon occurs.Furthermore, glycerin can decrease the slicing resistance while theetching speed suppressed from being increased, at a low concentration ofthe basic material as compared with that of other liquid. While thereasons therefor are not clear, it is considered that the presence ofglycerin increases the proportion of the basic material with respect towater to increase the effect (active amount) of the base. The suitableviscosity of glycerin decreases the diffusion rate of substances on thesilicon surface. While oxidation reaction is liable to proceed at theslicing part with the wire where the slurry flows vigorously, it isexpected that oxidation reaction, i.e., etching, is hard to proceed inthe part where the liquid flow on the wafer surface is small.

The first slurry for slicing a silicon ingot of the invention is aslurry for slicing a silicon ingot containing abrasive powder, a basicmaterial and water, in which the amount of the basic material is from 2to 6% by mass, and the amount of glycerin is from 25 to 55% by mass,based on the total mass of the components of the slurry excluding theabrasive powder.

The abrasive powder may be those generally used as an abrasive material,and examples thereof include silicon carbide, cerium oxide, diamond,boron nitride, aluminum oxide, zirconium oxide and silicon dioxide,which may be used solely or as a combination of two or more kindsthereof. The compounds capable of being used as the abrasive powder arecommercially available, and specifically, for example, silicon carbideis available under the trade names GC (Green Silicon Carbide) and C(Black Silicon Carbide) (produced by Fujimi Incorporated), and aluminumoxide is available under the trade names FO (Fujimi Optical Emery), A(Regular Fused Alumina), WA (White Fused Alumina) and PWA (PlateletCalcined Alumina) (produced by Fujimi Incorporated).

The average particle diameter of the abrasive powder is not particularlylimited, and is preferably from 5 to 20 μm. In the case where theaverage particle diameter of the abrasive powder is less than 5 μm, itis not preferred since the slicing speed becomes too small todeteriorate practicality, and in the case where the average particlediameter of the abrasive powder exceeds 20 μm, it is not preferred sincethe surface roughness of the wafer surface after slicing is increased todeteriorate the quality of the wafers in some cases.

The content of the abrasive powder is not particularly limited, and ispreferably from 40 to 60% by mass based on the total mass of the slurryfor slicing a silicon ingot. In the case where the content of theabrasive powder is less than 40% by mass, the slicing speed becomes toosmall to deteriorate practicality in some cases, and in the case wherethe content of the abrasive powder exceeds 60% by mass, the viscosity ofthe slurry becomes too large to impair supply of the slurry to theslicing interface in some cases.

The basic material may be such a substance that functions as a base inthe slurry, and examples thereof include an alkali metal hydroxide, suchas lithium hydroxide, sodium hydroxide and potassium hydroxide, and analkali earth metal hydroxide, such as magnesium hydroxide, calciumhydroxide and barium hydroxide, which may be used solely or as acombination of two or more kinds thereof. Among these materials, analkali metal hydroxide is preferably used from the standpoint ofreactivity with a silicon ingot.

A mixture of water and glycerin is used as the liquid component of theslurry. The water is preferably one having a small impurity content, butis not limited thereto. Specific examples thereof include pure water,extra-pure water, tap water and industrial water.

The slurry for slicing a silicon ingot can be prepared by mixing theaforementioned components at desired ratios. The method of mixing thecomponents may be arbitrarily selected, and for example, the componentsmay be agitated by a blade mixer. The order of mixing the component mayalso be arbitrarily selected. Furthermore, the slurry for slicing asilicon ingot thus prepared may be subjected to an additional process,such as a filtering process and an ion exchange process, for such apurpose as purification.

In the method for slicing a silicon ingot of the invention, a multi-wiresaw is used as a slicing device. In the slicing method, as having beendescribed, the first slurry of the invention is supplied to a slurrycoating head with a pump while agitating in an agitating tank, theslurry is applied from the slurry coating head to a wire that is woundand suspended on guide rollers in a large number of times and is movedat a high speed, and a silicon ingot is fed to the wire thus wound andsuspended. The abrasive powder are pressed onto the silicon ingot andsimultaneously are rotationally moved with the wire, whereby silicon isphysically removed as fine powder at the slicing interface, andsimultaneously, the removal attains with a small force with the chemicalaction of the basic material.

Examples of actual slicing operations using the first slurry for slicinga silicon ingot will be described in more detail as compared tocomparative examples.

Five kinds of slurries for slicing a silicon ingot as shown in Example 1and Comparative Examples 1 to 4 were produced, and silicon ingots weresliced under the following conditions to obtain results shown inTable 1. Upon producing the slurries, in all the cases, SiC abrasivepowder (GC #1500, having average particle diameter of about 8 μm,produced by Fujimi Incorporated) were used as the abrasive powder, andthe mass ratio between the abrasive powder and the other components thanthe abrasive powder in the slurry was 1/1. The viscosity of the slurrieswas adjusted to a value of from 50 to 130 mPa·s at a shear rate of 57.6(1/sec) and a slurry temperature of 25° C. The viscosity range wasobtained by a preliminary experiment as a suitable viscosity range uponslicing a silicon ingot with a multi-wire saw using an aqueous slurrycontaining abrasive powder.

(Slicing Conditions)

-   Slicing device: multi-wire saw (having a device constitution shown    in FIG. 1)-   Wire diameter: 0.1 mm (model SRH, produced by Japan Fine Steel Co.,    Ltd.)-   Abrasive powder: silicon carbide (GC #1500, produced by Fujimi    Incorporated, average particle diameter: about 8 μm)-   Silicon ingot: two ingots having a cross section of 150 mm square    and a length of 250 mm disposed-   Slicing pitch: 0.33 mm (kerf loss: 0.13 mm, wafer thickness: 0.2 mm)-   Slicing speed: 0.35 mm/min (ingot feeding speed)-   Wire running speed: 600 m/min-   Wire tension: 14 N-   Slurry tank temperature setting: 25° C.

Example 1

A mixed liquid of 40% by mass of glycerin, 56% by mass of water and 4%by mass of sodium hydroxide was produced, to which the same amount ofabrasive powder were then added, and the mixture was agitated.

Comparative Example 1

A mixed liquid of 39% by mass of propylene glycol, 1% by mass ofpolyvinyl alcohol, 56% by mass of water and 4% by mass of sodiumhydroxide was produced, to which the same amount of abrasive powder werethen added, and the mixture was agitated.

Comparative Example 2

A mixed liquid of 45% by mass of ethylene glycol, 51% by mass of waterand 4% by mass of sodium hydroxide was produced, to which the sameamount of abrasive powder were then added, and the mixture was agitated.

Comparative Example 3

A mixed liquid of 50% by mass of diethanolamine, 46% by mass of waterand 4% by mass of sodium hydroxide was produced, to which the sameamount of abrasive powder were then added, and the mixture was agitated.

Comparative Example 4

A commercially available neutral coolant (Luna Coolant #691, produced byOhtomo Chemical Co., Ltd.) was mixed with the same amount of abrasivepowder, and the mixture was agitated.

TABLE 1 Wire Slurry Wafer Wire Increase Fluctuation pulling Wear rate ofin Crack resistance rate viscosity thickness Surface depth Breakage (N)(%) (%) Solidification (μm) irregularity (μm) Example 1 none 0.8 ± 0.0714 20 none 11 none 0 Comparative found 1.1 ± 0.35 — — found — — —Example 1 Comparative found 1.2 ± 0.30 — — found — — — Example 2Comparative found 0.7 ± 0.20 13 250  found  8 medium 0 Example 3Comparative found 2.5 ± 0.26 23 40 none 35 small 10  Example 4

The presence of breakage of the wire in Table 1 shows a result obtainedby carrying out the silicon ingot slicing experiment in three times eachfor Example 1 and Comparative Examples 1 to 4. The slicing operation wascompleted in all the three experiments without breakage of the wire onlyin Example 1. In Comparative Examples land 2, the wire was broken untilthe silicon ingot of 150 mm square was completely sliced in all thethree experiments, and thus the wire pulling resistance and thesolidification state of the slurry that could be measure were onlyshown. In Comparative Examples 3 and 4 each, the wire was broken duringthe slicing operation in two experiments, and the wire was broken duringthe slicing operation of dummy glass, to which the silicon ingot wasadhered and fixed, in one experiment.

For Example 1 where wafers were obtained, the worst values among thethree experimental values were shown for the other evaluation items. ForComparative Examples 3 and 4, the measurement values of the experimentwhere wafers were obtained were shown. The wire wear rate means adecreasing rate of wire cross sectional area after completing theslicing operation of the silicon ingot with respect to the wire crosssectional area before use. The slurry viscosity increasing rate means anincreasing rate of the slurry viscosity after completing the slicingoperation of the silicon ingot with respect to the slurry viscositybefore slicing the silicon ingot. The presence of solidification of theslurry shows whether or not aggregates of the slurry are formed duringthe slicing operation of the silicon ingot.

The fluctuation in thickness of wafers is derived from two factors,i.e., the displacement of the wire upon slicing the silicon ingot andthe dissolution due to the etching action in the slicing terminationpart with the alkaline slurry. These factors were not distinguished fromeach other, and five wafers for each of both ends parts and center partof each of two ingots, i.e., 30 wafers in total, were sampled andmeasured for thickness at nine points per one wafer, i.e., four corners,intermediate points thereof, and center point. The standard deviationwas calculated from the data of 270 points in total. The surfaceirregularity of the wafers shows the extent of steps of minuteundulation and saw marks on the surface. The large irregularity meanspoor quality, and the no irregularity means such an extent that cannotbe found visually. The crack depth on the wafer surface layer is aresult of SEM observation of the cross section obtained by cutting thepart of the wafer surface that suffers relatively large irregularity.The crack depth of 0 μm means that no crack is found from the concavepart on the surface to the lower layer.

In Comparative Example 3 (alkaline slurry), the wire pulling resistanceof the wire was small as compared to Comparative Example 4 (neutralslurry) to obtain the similar effect as Example 1, but fluctuation inwire pulling resistance was large. Furthermore, the increasing rate ofthe viscosity of the slurry was large, and solidification of the slurryand separation between the solidified slurry and the liquid phase werefound. Comparative Example 3 provided a smaller fluctuation in waferthickness than Example 1, no crack in the surface layer as similarthereto, and a small wear rate of the wire, which were considered asgood properties, but the action of the abrasive powder was unstable uponslicing, which was found from the fact that the state of the slurry wasconsiderably unstable, and the fluctuation rate of the wire pullingresistance of the wire was large. In Example 1, the effect of the use ofthe alkaline slurry is obtained, and simultaneously, the stable slicingoperation was realized without breakage of the wire, so as to obtaingood results for all the evaluation items.

The component ratios capable of attaining a stable slicing operation ofa silicon ingot were investigated by changing the component ratios ofthe slurry of Example 1. Correlativity was found among the wire pullingresistance of the wire, the wear rate of the wire and the crack depth inthe surface layer of the wafer, and therefore, the presence of breakageand the wire pulling resistance of the wire, the increasing rate ofviscosity and the presence of solidification of the slurry, and theirregularity on the surface and the fluctuation in thickness of thewafers were used as the evaluation items. The evaluation methods for theitems were the same as those in Table 1. The mass ratio between theabrasive powder and the other components than the abrasive powder in theslurry was 1/1, and the same slicing experiments as above were carriedout with the contents of glycerin and sodium hydroxide changed. Theresults obtained are shown in Table 2.

TABLE 2 Ratios of components in mixed liquid excluding Wire Slurry Waferabrasive powder Wire Increase Fluctuation (% by mass) pulling rate of inSodium resistance viscosity Surface thickness hydroxide Glycerin WaterBreakage (N) (%) Solidification irregularity (μm) Comparative 1 40 59found 2.1 ± 0.36 73 found large 29 Example 5 Comparative 2 20 78 found1.8 ± 0.25 27 none large 23 Example 6 Example 2 2 25 73 none 1.6 ± 0.1524 none none 16 Example 3 2 55 43 none 1.5 ± 0.09 80 none none 14Comparative 2 60 38 none 1.5 ± 0.08 115 found small 20 Example 7Comparative 4 20 76 none 0.9 ± 0.35 38 found medium 16 Example 8Comparative 4 60 36 none 0.8 ± 0.07 105 found small 14 Example 9Comparative 6 20 74 none 0.7 ± 0.38 50 found large 15 Example 10 Example4 6 25 69 none 0.6 ± 0.20 48 none none 11 Example 5 6 55 39 none 0.7 ±0.15 94 none none 14 Comparative 6 60 34 none 0.7 ± 0.16 119 found none19 Example 11 Comparative 7 40 53 found 0.6 ± 0.35 139 found none 31Example 12

It is understood from Table 2 that the wire pulling resistance of thewire is increased when the content of sodium hydroxide is small. InComparative Example 5 (sodium hydroxide content: 1% by mass), no largedifference was obtained in wire pulling resistance of the wire fromComparative Example 4 (neutral slurry) in Table 1, and thus no effectwas obtained by using the alkaline slurry. In Comparative Example 12(sodium hydroxide content: 1% by mass), dissolution of the wafer in theslicing initiation part proceeded due to the too large action of alkali,and thus the fluctuation in thickness of the wafer was increased. Theother evaluation items thereof were also poor. It was considered that nosurface irregularity of the wafer was obtained by affection of thealkali etching action.

A large content of glycerin provides a relatively small wire pullingresistance of the wire with small fluctuation thereof, but a largecontent of glycerin provides a high increasing rate of the slurryviscosity.

A slurry having a large glycerin content had a large viscosity at thetime of preparation of the slurry, and thus the wire was not broken inComparative Examples 9 and 11. However, the increasing rates of slurryviscosity thereof exceeded 100%, and solidification (gelation) of theslurry was found, which showed unstable property of the slurry.

In Comparative Example 6, the average value of the wire pullingresistance of the wire was lower than that in Comparative Example 5 toprovide effect of the use of the alkaline slurry, but the fluctuation inwire pulling resistance was large, and the other evaluation itemsprovided poor results, which brought about an unstable slicing operationof a silicon ingot. A stable slicing operation of a silicon ingot wasobtained in Examples 2 and 3 having the same sodium hydroxide contentbut having a higher glycerin content, but in Comparative Example 7having a higher glycerin content, the increasing rate of slurryviscosity thereof exceeded 100%, and solidification (gelation) of theslurry was found, which showed unstable property of the slurry.

Solidification of the slurry was found in Comparative Examples 8, 9 and10. In Comparative Example 8, the surface irregularity of the wafer waslarge, and the wire pulling resistance of the wire was unstable. InComparative Example 10, the surface irregularity of the wafer wasunallowably large, and the wire pulling resistance of the wire wasunstable. Comparative Example 9 provided the best evaluation resultsamong Comparative Examples 5 to 12, but the increasing rate of slurryviscosity thereof exceeded 100%, which showed unstable property of theslurry. Examples 4 and 5 provided results that were equal to or betterthan Examples 2 and 3.

As having been described, in the case where a slurry contains a basicmaterial in an amount of from 2 to 6% by mass and glycerin in an amountof from 25 to 55% by mass, based on the total mass of the components ofthe slurry excluding the abrasive powder, the effect of the use of thealkaline slurry is obtained, and a stable slicing operation is realizedwithout breakage of a wire, so as to provide good results in all theevaluation items.

Embodiment 2

The second slurry for slicing a silicon ingot of the invention canconsiderably decrease unevenness in thickness of wafers for solar cellsby adding a small amount of a nonionic polymer surfactant to the slurrydescribed in Embodiment 1. As having been described, upon forming aslicing groove with a wire proceeding in the slicing direction, theslurry is continuously supplied to the slicing groove, wherebydissolution of the wafer in the direction perpendicular to the slicingdirection (the thickness direction of the wafer) proceeds by the etchingaction in the slicing initiation part as compared to the slicingtermination part of the ingot. As a result of investigations of variouskinds of surfactant in order to prevent the dissolution in the partwhere the slicing operation is completed without decrease in chemicalaction at the slicing interface, it has been found that the dissolutionin the part where the slicing operation is completed can be largelysuppressed through reduction in chemical action within the allowablerange by adding a suitable amount of a nonionic polymer surfactant.

A nonionic polymer surfactant was then added to the slurry of Example 1to verify the effect of suppressing the dissolution amount in theslicing initiation part of the wafer upon slicing a silicon ingot. As aresult of investigations of various kinds of surfactants by a beakerexperiment, Adeka Pluronic L31, produced by Adeka Corp. was optimum asthe nonionic polymer surfactant and was used. A mixture of 4% by mass ofsodium hydroxide and 40% by mass of glycerin, based on the total mass ofthe components of the slurry excluding the abrasive powder, in which theaddition amount of the nonionic polymer surfactant was increased (thesame amount of water was decreased), was added with SiC abrasive powder(GC #1500, produced by Fujimi Incorporated) in the same amount as thetotal components in the slurry excluding the abrasive powder, so as toproduce a slurry. The same slicing experiment as above was carried outto evaluate the wire pulling resistance of the wire and the fluctuationin wafer thickness. The results are shown in Table 3.

TABLE 3 Concentration of Wire pulling resistance Fluctuation in wafersurfactant (% by mass) of wire (N) thickness (μm) 0.1 0.8 ± 0.08 12 0.20.9 ± 0.08 8 0.4 1.3 ± 0.06 7 0.6 1.7 ± 0.05 6 0.8 2.0 ± 0.08 8 0.9 2.1± 0.10 10 1.0 2.3 ± 0.15 17

It is understood from Table 3 that the wire pulling resistance of thewire is increased upon increasing the concentration of the surfactant,and at 1.0% by mass, becomes such a value that is equivalent to thevalue obtained by using the neutral slurry in Comparative Example 4 tofail to obtain the effect of using the alkaline slurry, which shows thatthe limit of addition is 0.9% by mass. It is also understood that theeffect of suppressing the dissolution amount in the slicing initiationpart of the wafer upon slicing the ingot cannot be obtained at anaddition concentration of 0.1% by mass, and the effect is obtained at0.2% by mass.

As having been described, the second slurry for slicing a silicon ingotof the invention is a slurry for slicing a silicon ingot containingabrasive powder, a basic material and water, in which the amount of thebasic material is from 2 to 6% by mass, the amount of glycerin is from25 to 55% by mass, and the amount of a nonionic polymer surfactant isfrom 0.2 to 0.9% by mass, based on the total mass of the components ofthe slurry excluding the abrasive powder, whereby the dissolution in thepart where the slicing operation is completed can be suppressed withoutreduction in chemical action at the slicing interface.

Various modifications and changes may be applied to the invention by askilled person in the art without departing from the scope and spirit ofthe invention, and the invention is not construed as being limited tothe embodiments described in the specification.

1. A slurry for slicing a silicon ingot, comprising abrasive powder, abasic material and water, wherein the slurry comprises the basicmaterial in an amount of from 2 to 6% by mass and glycerin in an amountof from 25 to 55% by mass, based on a total mass of components of theslurry excluding the abrasive powder, wherein one or a combination oftwo or more of silicon carbide, cerium oxide, diamond, boron nitride,aluminum oxide, zirconium oxide and silicon dioxide is used as theabrasive powder.
 2. A slurry for slicing a silicon ingot, comprisingabrasive powder, a basic material and water, wherein the slurrycomprises the basic material in an amount of from 2 to 6% by mass,glycerin in an amount of from 25 to 55% by mass, and a nonionic polymersurfactant in an amount of from 0.2 to 0.9% by mass, based on a totalmass of components of the slurry excluding the abrasive powder.
 3. Theslurry for slicing a silicon ingot as claimed in claim 1, wherein theabrasive powder has an average particle diameter of from 5 to 20 μm, anda content of the abrasive powder is from 40 to 60% by mass based on atotal mass of the slurry for slicing a silicon ingot.
 4. The slurry forslicing a silicon ingot as claimed in claim 1, wherein an alkali metalhydroxide or an alkali earth metal hydroxide is used as the basicmaterial.
 5. A method for slicing a silicon ingot with a slurrycomprising abrasive powder and a basic material, wherein the slurrycomprises the basic material in an amount of from 2 to 6% by mass andglycerin in an amount of from 25 to 55% by mass, based on a total massof components of the slurry excluding the abrasive powder, wherein oneor a combination of two or more of silicon carbide, cerium oxide,diamond, boron nitride, aluminum oxide, zirconium oxide and silicondioxide is used as the abrasive powder.
 6. A method for slicing asilicon ingot with a slurry comprising abrasive powder and a basicmaterial, wherein the slurry comprises the basic material in an amountof from 2 to 6% by mass, glycerin in an amount of from 25 to 55% bymass, and a nonionic polymer surfactant in an amount of from 0.2 to 0.6%by mass, based on a total mass of components of the slurry excluding theabrasive powder.
 7. The method for slicing a silicon ingot as claimed inclaim 5, wherein a multi-wire saw is used as a slicing device to producewafers for solar cells.
 8. The slurry for slicing a silicon ingot asclaimed in claim 2, wherein one or a combination of two or more ofsilicon carbide, cerium oxide, diamond, boron nitride, aluminum oxide,zirconium oxide and silicon dioxide is used as the abrasive powder. 9.The slurry for slicing a silicon ingot as claimed in claim 2, whereinthe abrasive powder has an average particle diameter of from 5 to 20 μm,and a content of the abrasive powder is from 40 to 60% by mass based ona total mass of the slurry for slicing a silicon ingot.
 10. The slurryfor slicing a silicon ingot as claimed in claim 2, wherein an alkalimetal hydroxide or an alkali earth metal hydroxide is used as the basicmaterial.
 11. The method for slicing a silicon ingot as claimed in claim6, wherein a multi-wire saw is used as a slicing device to producewafers for solar cells.